The present invention provides a drive system for a multi-phase brushless motor comprising a plurality of phases, the system comprising a drive circuit including switch means for varying the electric potential applied to each of the phases by switching the drive circuit between a plurality of states, a current sensor connected such that it can sense the instantaneous total current flowing through the windings, and control means arranged to provide pulse width modulated drive signals to control the switch means so as to control the time that the drive circuit switches between said states in each of a series of pulse width modulation periods, and to:                determine a demanded voltage parameter set,        identify PWM periods during which the demanded voltage parameter set is such that neither two nominal corresponding state times, nor a higher number of equivalent state times producing the same net voltage, in a single PWM period, would allow a predetermined minimum time to be spent in a predetermined number of active states sufficient for the current in each of the phases to be determined by means of the current sensor,        for such PWM periods, to calculate modified state times that do allow sufficient time to be spent in a sufficient number of active states for the current in each of the phases to be determined by means of the current sensor.        
The state times may be defined in terms of state vectors. In this case they may be modified by modifying the states vectors. Alternatively they may be defined in terms of switching times at which the drive current is switched between states. In this case they can be modified by identifying the switching times.
The demanded voltage parameter set may include only a single parameter, such as a modulation index, or may include two or more parameters, such as alpha and beta voltage demands.
The present invention further provides a drive system for a multi-phase brushless motor comprising a plurality of phases, the system comprising a drive circuit including switch means for varying the electric potential applied to each of the phases by switching the drive circuit between a plurality of states, a current sensor connected such that it can sense the instantaneous total current flowing through the windings, and control means arranged to provide pulse width modulated drive signals to control the switch means so as to control the time that the drive circuit switches between said states in each of a series of pulse width modulation periods, and to:                determine a demanded voltage parameter set,        for some values of the demanded voltage parameter set, identify two states which could, if selected for nominal state times, achieve the demanded voltage parameter set, and        define modified state times for the same two states, one being of said predetermined minimum time, and the other being a predetermined maximum time.        
Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.